Last August, looking up from the edge of an ancient river delta on Mars, the Perseverance rover’s electronic eye spied an oblong shadow crossing the distant, glowing Sun. Phobos, the Martian moon named for the Greek god of fear and panic, was transiting the solar disk. Too small to darken the pale, dust-tinged sky, but orbiting close enough to eclipse a significant fraction of the Sun, Phobos passed in less than a minute.

NASA’s Perseverance rover captured Phobos passing in front of the Sun from the surface of Mars.ASU/MSSS/SSI/JPL-Caltech/NASA

The tiny moon, with its murky origins, is an enigma, and one whose makeup and history could tell us much about Mars and the wider solar system. And now the international Martian Moons eXploration (MMX) mission, which successfully launched on 20 October, seeks to explore that mystery with an unprecedented series of landings on the unknown surface of Phobos.

This mission brings unusual challenges—for example, designing an orbit to study a microgravity moon orbiting a much larger planet. Engineers also had to figure out how to assess and land on this low-gravity pile of loosely assembled dirt and rock, how a rover might rove across it, and how the spacecraft might take samples and return them to Earth.

“I often say that landing on Phobos is like trying to land a balloon,” says Yasuhiro Kawakatsu , project manager of MMX at the Japan Aerospace Exploration Agency (JAXA).

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